tests: avoid tons of spurious failures on NetBSD * t/ax/am-test-lib.sh (process_requirements): Set the '$am_tool' variable to the empty string before trying to unset it; otherwise, we might be attempting to unset an already-unset variable, which (together with the presence of the 'errexit' shell flag) causes spurious failures at least with the /bin/sh shell from NetBSD 5.1. This was actually causing the great majority of the Automake tests (all those not using a "required=..." declaration) to fail spuriously on that platform! Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>